1. Field of the Invention
The present invention pertains generally to the field of radiation imaging. More specifically, the present invention pertains to the field of detectors for scanning beam x-ray imaging systems.
2. Description of Related Art
Real-time x-ray imaging is increasingly being required by medical procedures as therapeutic technologies advance. For example, many electro-physiologic cardiac procedures, peripheral vascular procedures, PTCA procedures (percutaneous transluminal catheter angioplasty), urological procedures, and orthopedic procedures rely on real-time x-ray imaging. In addition, modern medical procedures often require the use of instruments, such as catheters, that are inserted into the human body. These medical procedures often require the ability to discern the exact location of instruments that are inserted within the human body, often in conjunction with an accurate image of the surrounding body through the use of x-ray imaging.
A number of real-time x-ray imaging systems are known. These include fluoroscope-based systems where x-rays are projected into an object to be x-rayed and shadows caused by relative x-ray opaque matter within the object are displayed on the fluoroscope located on the opposite side of the object from the x-ray source. An example of a known fluoroscopy system is U.S. Pat. No. 2,730,566 issued to Bartow, et al. entitled "Method and Apparatus for X-Ray Fluoroscopy."
Reverse-geometry scanning-beam x-ray imaging systems are also known. In such systems, an x-ray tube generates an electron beam which is focussed upon a small spot on the relative large anode (transmission target) of the tube, inducing x-ray radiation emission from that spot. The electron beam is deflected (electromagnetically or electrostatically) in a scanning pattern over the anode. A small x-ray detector is placed at a distance from the anode of the x-ray tube. The detector typically converts x-rays which strike it into an electrical signal in proportion to the detected x-ray flux. When an object is placed between the x-ray tube and the detector, x-rays are attenuated by the object in proportion to the x-ray density of the object. While the x-ray tube is in the scanning mode, the signal from the detector is inversely proportional to the x-ray density of the object.
The spatial resolution and the signal-to-noise ratio of x-ray images formed by known reverse-geometry scanning x-ray imaging systems are dependent, to a large extent, upon the size of the sensitive area of the detector. If the detector aperture is increased in area, more of the diverging rays are detected, effectively increasing sensitivity and improving the signal-to-noise ratio. At the same time, however, the larger detector aperture reduces attainable spatial resolution as the "pixel" size (measured at the plane of the object to be imaged) becomes larger. This is necessarily so because most objects to be imaged in medical applications (e.g., structures internal to the human body) are some distance from the x-ray source. In the known systems, therefore, the detector aperture size has been selected so as to effect a compromise between resolution and sensitivity, it not being previously possible to maximize both resolution and sensitivity simultaneously.
Examples of known reverse-geometry scanning-beam x-ray systems include those described in U.S. Pat. Nos. 3,949,229 to Albert; 4,032,787 to Albert; 4,057,745 to Albert; 4,144,457 to Albert; 4,149,076 to Albert; 4,196,351 to Albert; 4,259,582 to Albert; 4,259,583 to Albert; 4,288,697 to Albert; 4,321,473 to Albert; 4,323,779 to Albert; 4,465,540 to Albert; 4,519,092 to Albert; and 4,730,350 to Albert.
Accordingly there is a need for an x-ray detector which contains a large enough detection area to provide high detection sensitivity while containing sufficiently sized detection elements to maintain increased spatial resolution.